For purposes of the present discussion, the capacity on the first discharge of the MnO.sub.2 contained in the positive electrode between the MnO.sub.2 status and the MnOOH status is termed or designated as the theoretical one electron discharge capacity of the manganese dioxide electrode, which is 308 m Ah/g of manganese dioxide. If the discharge process of the MnO.sub.2 positive electrode continues beyond the MnOOH level, an irreversible phase change has been reported to occur, so that the manganese dioxide electrode is no longer fully rechargeable.
Specifically, the following equation is descriptive of the discharge reaction which takes place as the MnO.sub.2 discharges towards its MnOOH status in the presence of an aqueous electrolyte. EQU MnO.sub.2 +H.sub.2 O+e.sup.- =MnOOH+OH.sup.-
Discharge according to this reaction occurs with essentially no phase change. If the discharge process of the manganese dioxide positive electrode continues beyond the MnOOH level, an irreversible phase change has been reported to occur, so that the manganese dioxide electrode is no longer dully rechargeable. Such phase change must be avoided. Responsible for the phase change is the following reaction: EQU MnOOH+H.sub.2 O+e.sup.- =Mn(OH).sub.2 +OH.sup.-
However, particularly with reference to practical aqueous manganese dioxide/zinc cells, discharge according to this reaction occurs at a voltage which is too low to contribute to the useful service life of the cell, since it occurs below 0.8 volts.
To provide rechargeable alkaline manganese dioxide/zinc cells, there have been a number of steps taken to ensure rechargeability; and specifically, steps have been taken to severely limit the discharge of the MnO.sub.2 to a fraction of the one electron discharge by limiting the capacity of the zinc electrode accordingly.
Historically, rechargeable alkaline MnO.sub.2 /Zn cells that have been brought to the market in the late 1960's and early 1970's were not successful because of the constraints placed upon them. Those constraints included the use of electronic controls to cut off the discharge after a certain time of use to not permit discharge beyond the MnOOH level. In general, such cells were merely modified primary alkaline manganese dioxide/zinc cells, and generally they had the same ratio of Zn metal in the negative electrode to MnO.sub.2 in the positive electrode as primary cells and, therefore, had a very limited cycle life. Once a battery of cells in series had been discharged to 0 Volt, the cells were no longer rechargeable. Such cells were also quite low in respect of their energy densities: For example, a rechargeable D cell may have been rated at only 2 Ah. A more full discussion of the above is found in FALK and SALKIND Alkaline Storage Batteries, published by John Wiley & Sons, New York, 1969, at pages 180 to 185, and also pages 367 to 370.
So as to overcome the recharge problems of the MnO.sub.2, as noted above, cells were developed by which the discharge capacity of the cell was limited by imposing a zinc negative electrode limitation--by which it was made impossible to discharge the MnO.sub.2 to more than a fraction of the one electron discharge capacity of the negative electrode. For example, the discharge capacity of the zinc negative electrode was limited by design to be no more than about 30% of the one electron discharge capacity of the MnO.sub.2 in the positive electrode. This preserved the rechargeable characteristics of the cell, but resulted in a cell having quite low deliverable energy capacity and density. Those limitations, understandably, mitigated against the commercial acceptability of such cells.
Reference is made to AMANO et al U.S. Pat. No. 3,530,496, issued Sep. 22, 1970. AMANO et al make a very strong statement of their intent to limit the depth of discharge of the MnO.sub.2 electrode by providing a zinc electrode that has its capacity limited to between 20% to 30% of the one electron discharge capacity of the MnO.sub.2 in the positive electrode. It should be noted that the concept of electrode balance is well known, and for the cells discussed it is the ratio of the theoretical discharge capacity of the metallic zinc in the negative electrode with respect to the theoretical one electron discharge capacity of the manganese dioxide in the positive electrode. AMANO et al shows, in FIG. 8 of that patent, the capacity decay vs. cycle of "D" size cells in cases where the depth of discharge of the MnO.sub.2 electrode is set at 100%, 50%, 30%, 20% and 10% of the theoretical one electron MnO.sub.2 discharge capacity. The target depth of discharge of the MnO.sub. 2 electrode is set by varying the theoretical capacity of the negative electrode. All "D" size test cells were cycled until their discharge capacity fell below 0.5 Ah, that is below an energy density of about 4 Wh/kg of cell weight. Using a 50% electrode balance, AMANO et al report an initial cell capacity of 4.5 Ah, which degrades to 0.5 Ah within 15 cycles. In other words, the 15th cycle capacity represents 11% of the first cycle capacity. Based on the negative findings of this experiment, AMANO et al conclude that an electrode balance exceeding 30% of the theoretical one electron MnO.sub.2 discharge capacity produces a cell with a cycle life which is too low to be commercially viable. In their Table 1, AMANO et al list the number of cycles achieved in their experiments to an end of life discharge capacity of 0.5 Ah--that is, 3.6 Ah/kg--as a function of the cell balance. They report the service life of the cell as being only 6 cycles at 100% balance, 15 cycles at 50% balance, 35 cycles at 30% balance, 84 cycles at 20% balance, and over 99 cycles at 10% balance. These results have led AMANO et al to the conclusion that an electrode balance exceeding 30% of the theoretical one electron MnO.sub.2 discharge capacity results in a cell that has poor performance characteristics.
OGAWA et al, in U.S. Pat. No. 3,716,411, issued Feb. 13, 1973, teach a rechargeable alkaline manganese cell with the discharge capacity of the zinc electrode controlled within a range so as to ensure the recharge capability of the MnO.sub.2 electrode. The zinc electrode and MnO.sub.2 electrode face each other through a gas permeable and dendrite impermeable separator. However, the OGAWA et al cell is strictly negative electrode limited in that the capacity of the zinc electrode is held to be not more than about 40% of the theoretical one electron discharge capacity of the manganese dioxide in the positive electrode. OGAWA et al discuss the fact that if a manganese dioxide/zinc cell is discharged to below 0.9 volts and down to about 0.75 volts, and where the capacity of the zinc negative electrode is about the same or slightly smaller than that of the manganese dioxide positive electrode, then the recharge capability of the manganese dioxide strongly deteriorates. OGAWA et al provide that under no conditions should the depth of discharge of the zinc electrode be permitted to exceed 60% of the theoretical one electron discharge capacity of the manganese dioxide positive electrode.
KORDESCH, in U.S. Pat. No. 4,091,178, issued May 23, 1978, also provides a rechargeable MnO.sub.2 /Zn cell where the theoretical discharge capacity of the zinc negative electrode is specifically limited to about 33% of the one electron discharge capacity of the positive electrode. However, KORDESCH also provides what he calls a "charge reserve mass", in which a quantity of zinc oxide is placed that is equal to at least 50% of the discharge capacity of the metallic zinc in the negative electrode. The energy density of the KORDESCH cell is quite low.
Tomantschger et al, in a commonly owned U.S. patent application Ser. No. 07/893,793 filed Jun. 4, 1992, provide rechargeable alkaline manganese zinc cells that utilize an MnO.sub.2 positive electrode and a zinc negative electrode, wherein the discharge capacity of the zinc electrode is limited to from greater than 60% and up to 100% of the theoretical one electron discharge capacity of the MnO.sub.2. That provides a rechargeable alkaline manganese cell having higher capacity and higher energy density than has been available from the prior art cells.